16 December 2010

"The demo code provides a Google APIV3 Custom Map Type for OS OpenSpace tiles. There is no reason why the same approach can not be used to warp tile sets from other, non Google projection tile servers. The basic affine transform technique could also be used to provide an OS OpenSpace Custom Overlay for Google Maps. This might use less DOM nodes but would not easily support the hidden peripheral tile fetching of the Custom Map Type.

The demo code uses the WGS84 Lat/Lon to OS East/North coordinate conversion from OSGB Web Map Tools and the Ground Control Point to Affine Transfrom Matrix algorithm from Google Mapping Hacks. The coordinate conversion accuracy is 1-2m across the UK, consistent with the 1m pixels used by the most zoomed in OS mapping. Whilst the OS OpenSpace tile server uses WMS like URLs, I do not believe that it will return Google projection tiles.

The demo only works in SVG enabled browsers. I have tried it in Firefox 3.6, Safari 5.02, Opera 10.63 and Chrome 8.0.552.215 and Internet Explorer 9 beta. Chrome seems to be the fastest and gives the best visual results (invisible joins between OS tiles)."